Uranus
is the third of the gas giants. It is different from the other gas giants.
It is tilted on its side, it doesn't make much of its own internal heat,
and one of its moons has the tallest cliff in the whole solar system -
some 20 km high.

Uranus, the 7th planet out
from the Sun was discovered accidentally in 1781 when William Herschel
was trying out the 7" telescope that he had built. It is barely visible
to the naked eye, but through a telescope it looks like a blue-green disc.

It's about four times the size
of Earth. Uranus has a solid core made up of iron
and silicates. This solid core is about 14,000 km in diameter, just a
little larger than our whole planet. Above the solid core is a 9,000 km
thick layer of ice and various gases (mostly hydrogen and helium, with
a little methane).

There doesn't seem to be much
visible weather happening on Uranus. The cloud tops are almost featureless.
Another peculiarity about the weather of Uranus is that the temperature
from the Poles to the equator is almost constant, within 2oC.

The planet is tilted, and lies
on its side. So in its 84 year orbit, the Sun shines
on one Pole for 42 years, and then on the other pole for another 42 years.
The current scientific theory is that a colossal impact with a pretty
large body must have tipped Uranus on its side. This must have happened
fairly early in the history of Uranus. The satellites go around the equator
of Uranus, so they must have formed after the big impact that tilted Uranus
onto its side. The day is shorter than our own - about 17-18 hours long.

In 1977, some five rings were
discovered around Uranus. But when Voyager 2 zipped past, it discovered
another four rings. These rings are very dark and narrow.

Before Voyager 2, only 5 moons
were known. But Voyager 2 discovered another 10 moons. Two of them are
tiny "shepherd moons", like the shepherd moons that maintain the braided
rings of Saturn.

But the real surprise was the
moon Miranda. It's a small moon, only about 350 km across. It's made half
of water ice, with the other half being rocks. Yet this tiny moon has
huge deep oval scratches, about 200-300 km across. It also has the tallest
cliffs in the known solar system - 20 km high.